Preparation and conformational analysis of 6,10-diethyl[1,2,3]trithiolo-[4,5-h]benzopentathiepin monoxides:: Isolation and optical properties of chiral benzopentathiepin derivatives

6,10-Diethyl[1,2,3]trithiolo[4,5-h]benzopentathiepin (1) was oxidized by mCPBA in dichloromethane to produce the corresponding four monoxides. The structures of the monoxides 2, 3, 4, and 5 were determined by X-ray crystallography to be 6,10-diethyl[1,2,3]trithiolo[4,5-h]benzopentathiepin 8-oxides (1,2,3-trithiole 2-oxides) for 2 and 3, and 6,10-diethyl[1,2,3]trithiolo[4,5-h]benzopentathiepin 7-oxides (1,2,3-trithiole 1-oxides) for 4 and 5, respectively. Compounds 2 and 3 are isomers with respect to the conformation of the 1,2,3,4,5-pentathiepin ring, and 4 and 5 are also the conformational isomers. These compound pair members, 2 and 3, and 4 and 5, were found to isomerize each other by inversion of the pentathiepin ring. The activation parameters of the isomerization, DeltaG(298)(not equal), DeltaH(not equal), and DeltaS(not equal), were determined by H-1 NMR spectroscopy. Because of the slow inversion of the pentathiepin ring, isolation of unsymmetrically substituted benzopentathiepin as a chiral molecule is possible. Asymmetric oxidation of I was performed by a Sharpless reagent [1/Ti((OPr)-Pr-l)(4)/R,R-DET/t-BuOOH = 1: 2:4:4] to produce optically active monoxides 4a and 5a. The configuration of 4a and 5a was confirmed as R configuration on the sulfinyl sulfur atom, respectively. The specific rotation and the circular dichroism spectra of 4a and 5a were measured in chloroform; such data are apparently affected by the conformation of the pentathiepin ring.